Vacuum pumps typically comprise a body and a rotor supported for rotation relative to the body to draw gas from a tool connected to the inlet of the pump. The rotor is supported by a bearing arrangement comprising two bearings located at or intermediate respective ends of the rotor. Usually, the upper bearing is in the form of a magnetic bearing, and the lower bearing is in the form of a rolling bearing.
As illustrated in FIG. 1, a typical rolling bearing 10 comprises an inner race 12 extending about the rotor 14, an outer race 16, and a plurality of rolling elements 18, supported by a cage 20, for allowing relative rotation of the inner race 12 and the outer race 16. The rolling bearing 10 is lubricated to establish a load-carrying film separating the bearing components in rolling and sliding contact in order to minimize friction and wear, and shield elements 22 are provided to resist seepage of lubricant from the rolling bearing 10. A radial damping ring 24 is positioned radially between a radial end surface of the outer race 16 and the body 26 of the pump for damping radial movement of the outer race 16. An axial damping ring 28 is provided between an axial end surface of the outer race 16 and the body 26 for damping axial movement of the outer race 16. The rolling bearing 10 is secured to the rotor 14 by a threaded nut 30 which is screwed on to the end of the rotor 14 so that the upper (as illustrated) axial end surface 32 of the inner race 12 engages an abutment surface 34 of the rotor 14.
The upper magnetic bearing (not shown) typically comprises a stack of magnetic bearing rotor rings mounted on the rotor 14, and a stack of magnetic bearing stator rings, concentric with and located inside the rotor rings, mounted on an axially adjustable mounting which is accessed through the inlet of the pump. The axial position of the mounting is adjusted so that the stacks of rings are axially offset. Due to the forces of repulsion between the rings, the rotor 14 is biased in the axial direction so that an axial preload is applied to the rotor 14.
The rolling bearing 10 and the damping rings 24, 28 are usually replaced when the pump is serviced. As there is a tolerance stack-up between the upper (as illustrated) axial end surface of the inner race 12 of the bearing and the upper axial end surface 36 of the axial damping ring 28, this can result in the rotor 14 being in a different axial position following the replacement of these components. A change in this position of the rotor 14 will change the axial preload applied to the rotor 14 by the magnetic bearing; if this preload is too high the rolling bearing 10 may be subject to excessive wear, whilst if this preload is too low components of the rotor 14 may clash with components of the pump body 26 during use of the pump. Consequently, once the rolling bearing 10 has been replaced, the pump has to be disconnected from the tool so that the mounting for the magnetic bearing stator rings can be adjusted to ensure that the axial preload is at the required value. This can considerably increase the time required to service the pump.